(1) Field of the Invention
This invention relates to a three-way switching valve, and more particularly to a pilot-operated three-way switching valve for switching a flow path of a fluid through electromagnetic operation.
(2) Description of the Related Art
Conventionally, a three-way switching valve is known which operates to cause selective flow of a fluid supplied from a pump or the like toward two outlets. The three-way switching valve has two valve seats, and operates such that when one of the valve seats is closed, the other is opened. The opening and closing operations of the valve are carried out by actuating valve elements by a solenoid or a motor, or by using the difference between fluid pressures. Some three-way switching valves of the solenoid-operated type are configured to have two solenoids for actuating two valve elements alternately. Further, some of the type making use of the difference between pressures are required to introduce the difference between pressures on the discharge side and the suction side of a pump into a valve element or a portion for actuating the valve element, and some of the conventional three-way switching valves generate the differential pressure by connecting a capillary tube to the suction side of a pump.
On the other hand, a three-way switching valve is known e.g. from Japanese Unexamined Patent Publication No. 4-312276, which is configured to have one solenoid, and at the same time no external tube is provided so as to make its valve mechanism compact in size. The three-way switching valve forms a three-way solenoid valve having a solenoid and two valve elements arranged on the same axis and using a diaphragm pilot mechanism. When the solenoid is in a deenergized state, a pilot valve element formed on a plunger closes a pilot valve hole by the spring force, whereby a valve element on the solenoid side is closed, and a valve element on an opposite side of the solenoid is opened. Inversely, when the solenoid is energized, the pilot valve element is opened whereby a pressure in a diaphragm chamber is relieved to the downstream side of the valve to be opened, to thereby open the valve element on the solenoid side and at the same time close the valve element on the opposite side of the solenoid by the supplied fluid pressure.
However, the three-way switching valve constructed as above has a problem in durability especially when it is used for switching flow of a fluid having a very high pressure, since a diaphragm is employed in the pilot mechanism. In the three-way switching valve, to open and close the two valve elements in an interlocked fashion, a drive shaft is arranged such that it extends through a partition wall that separates two outlet passages. This causes a very small amount of leakage of the fluid through a portion slidably holding the drive shaft.